Mapping Water Quality Improvement Opportunities in Iowa
By Sam Blair and Beck Woollen, Agriculture Interns
Nutrient pollution from point and nonpoint sources threatens America’s water quality. Nitrogen and phosphorus impair around two-thirds of the U.S.’ coastal areas and over a third of its estuaries. Perhaps the best example of this pattern is the Gulf of Mexico, which has a hypoxic “dead” zone of over 5,000 square miles in size. Nutrient-rich runoff from states throughout the Mississippi River Basin contributes to the problem, but Iowa is a leading contributor. Iowa has a particularly acute nutrient pollution issue—with over 50% of its water bodies impaired. In response to this problem, Iowa set targets to reduce nitrogen wastewater pollution by 66% from baseline levels and phosphorus wastewater pollution by 75% from baseline levels. Our project illustrates which watersheds and cities are closest to meeting these reduction goals from municipal point sources, information critical to effectively targeting policies and resources to improve Iowa’s water quality.
Sam's Methods Before getting into the analysis itself, I first had to clean the dataset provided to make and find the precise locations of each pollution site. I used Python for my analysis, taking advantage of the geopandas package, which gives Python the ability to read the shapefiles used in more traditional GIS programs. For the map itself, I used KeplerGL, a high-quality open-source mapping that works well with Python and larger datasets. Presentation-wise, I choose to declutter the map, giving the user many different options for how to format the map based on the variable they want to examine or the appearance they desire.
Beck’s Methods Like Sam, my map-building process began with data cleaning. Sam generously provided me with latitude and longitude coordinates for the municipalities in our dataset, which significantly sped up the process. I calculated the percentage of nitrogen and phosphorus that still had to be reduced by each municipality and then uploaded the polished dataset to ArcGIS Pro. Using the Coordinate Table to Point and Spatial Join tools on ArcGIS Pro, I was able to plot these municipalities on a map with HUC8 watersheds. After merging these different layers of the map, I calculated an average (and per-capita) amount of nitrogen and phosphorus that had to be reduced within each HUC8. Finally, I used a gradient to color-code the municipalities and HUC8s across the state, illustrating where nitrogen and phosphorus reductions are most urgent.
Clarifications About Our Maps There are several key points to remember when studying these maps. To begin, they each include total remaining nitrogen reduction needs and per-capita values. Although these values are considerably related, they tell slightly different stories—think about the follies of comparing highly urbanized areas with mostly rural areas without any consideration of population size. Although Iowa’s nitrogen and phosphorus reduction targets are in terms of total pounds, the per-capita values can offer preliminary insights into the magnitude of change that individuals within a municipality or HUC8 will experience as reduction progresses.
What do our maps show? Firstly, they tell us the point sources of pollution which are farthest from reaching their nutrient pollution targets. Nitrogen pollution is particularly severe in the Apple-Plum subbasin of eastern Iowa, particularly in the city of Dubuque. Dubuque must eliminate about 2.1 million pounds of nitrogen to reach the target of a 66% reduction in raw nitrogen runoff. Council Bluffs is another problematic area: it is almost 300,000 pounds away from meeting the nitrogen pollution reduction target according to one of our calculations. In terms of phosphorus, Cedar Rapids has a particularly long way to go before hitting the 75% phosphorus reduction target; by one of our calculations, this municipality needs to cut down by over 3 million pounds of phosphorus. Iowa can use this information to help decide which cities and watersheds deserve the most attention, allowing for Iowa to better target policy interventions at locales that stand in most need, a necessity given the vast heterogeneity in water pollution levels across the state our maps reveal.
What’s Next These mapping techniques could be extended in several directions. Firstly, one could add information about pollution from industrial sources to these maps, which currently only account for pollution from public wastewater facilities and thus may not give a complete picture of point source water pollution in Iowa. Secondly, one could easily extend these techniques to other states like Illinois and Wisconsin, provided that one has adequate data on the sources of nutrient pollution in those states. Policy-wise, these maps tell us the places where water quality organizations and agencies’ work is most needed. Locales that are farthest from meeting their nutrient pollution reduction goals could be targeted as places where water quality partnerships— which help point pollution sources to meet their discharge targets by encouraging nearby farms to invest in conservation practices—could make the biggest difference. Agencies could investigate if successful cities, such as the Iowa City area, have particular techniques they use to ensure high water quality, and if they do, attempt to apply those methods to less successful cities.